Pb1−xCaxTiO3 thin films prepared by laser ablation of ceramic targets

Pb1−xCaxTiO3 thin films prepared by laser ablation of ceramic targets

surface EL+%VIER Applied Surface Science 96-98 science (1996) 823-826 Pb, _,Ca,TiO, thin films prepared by laser ablation of ceramic targets M.J...

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surface EL+%VIER

Applied Surface

Science 96-98

science

(1996) 823-826

Pb, _,Ca,TiO, thin films prepared by laser ablation of ceramic targets M.J. Martin, C. Zaldo

*,

J. Mendiola

Institute de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Cantoblanco, 28049 MadridvSpain Received 22 May 1995

Abstract Pb,_,Ca,TiO, thin films with thickness in the 200-1000 nm range have been prepared by pulsed laser deposition on Pt/TiO,/SiO,/(lOO)Si substrates. 10% weight PbO-rich ceramic targets have been ablated with a KrF excimer laser. It has been found that the Ca concentration of the films remains close to its concentration in the target (X = 0.24). Optimized crystalline orientation is obtained using non sintered targets, on the other hand optimized tetragonality has been found in 600 nm thick films. The films show an asymmetric ferroelectric behavior.

formed. Moreover the ferroelectric response of the films obtained has been characterized.

1. Introduction Calcium ions may be incorporated to PbTiO, in a large solid solution extend yielding Pb,_,Ca,TiO, ceramics. In bulk ceramics the increase of Ca reduces the lattice tetragonality (c/a) though a steady state value is reached for x > 0.33 [l]. For x = 0.24 bulk ceramics show optimized piezo and pyroelectric properties [1,21. Pulsed laser deposition (PLD) is a suitable technique to prepare ferroelectric thin films of PbTiO, and other related compounds, namely PLT and PLZT, the work performed have been recently reviewed [3]. In this work we have studied the Ca incorporation to crystalline PbTiO, films prepared by PLD paying particular attention to the relation between the target preparation procedure and the quality of the films

* Corresponding 3720623.

author. Tel.:

+34-l

3349057;

fax:

+34-l

2. Experimental

techniques

The beam of a Lambda Physik KrF excimer laser, model 105 (A = 248 nm, 300 mJ/pulse) was focused to ablate the ceramic target. The target is rotated at 20 ‘pm in a turbo-pumped vacuum cham ber. Oxygen is dynamically flowed to the chamber during deposition to choose a deposition pressure in the range 5 X 10m6-2 X 10-l mbar. Substrates are heated with a quartz lamp up to 600°C. The temperature is measured with a S-type thermocouple glued with silver paint close to the substrate. Pt/TiO,/SiO,/(lOO)Si substrates are held 4 cm away from the target. The target an film stoichiometry have been investigated by electron microprobe analysis using a

0169-4332/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved. SSDZ 0169-4332(95)00590-O

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M.J. Martin et al./Applied Surface Science 96-98 (1996) 823-826

Philips XL-30 equipment with a Si(Li) X-ray detector. The crystalline quality of the films has been investigated by grazing X-ray diffraction using a D-500 Siemens diffractomer operating at the K, emission of a Cu anode. Hysteresis measurements have been performed using a Radiant Technology RT66A Standardized Ferroelectric Test System. For this purpose gold dots are used as top electrode.

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3. Experimental

results and discussion

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Temperature Previous results on PLD of PbTiO, films indicate that the lead concentration in the films is lower than in the target [3]. To compensate for the expected Pb loss, we prepared 10% weight PbO-rich ceramics of Ca-modified PbTiO,. Two different procedures were used to prepare the ceramic targets: (a) 1050°C sintered ceramics using a procedure described previously [4]. These ceramics contains small Co, W and Mn amounts for densification purposes. (b) Non sintered ceramics were formed by mixing PbO, TiO, and CaCO, dried powders, the mixture is heated at 600°C pressed uniaxially up to 300 kg/cm* and isostatically up to 2000 kg/cm*, finally the resulting pellet is heated 6 h at 600°C. The [Pb]/[Ti] and [Ca]/[Ti] molar concentration ratios of the films calculated by microprobe analysis have been summarized in Figs. 1 and 2. Fig. 1 shows the evolution of the Pb and Ca concentration of the film for increasing substrate temperatures. For substrate temperatures lower than 400°C the Pb concentration in the film relative to the Ti concentration is higher than in the target, likely amorphous PbO is contributing to the average Pb concentration of the film. In agreement with previous work on PbTiO,, a marked decrease of the Pb concentration is observed for substrate temperatures above 400°C. On the other hand the Ca concentration has been found constant and equal to the Ca target concentration. For the target stoichiometry (10% Pb-rich) and oxygen pressure (1 X 10V2 mbar) used, the desired film stoichiometry (x = 0.24) is found heating the substrate at about 550°C. The lead loss is due to its reevaporation from the hot target. This may be minimized increasing the oxygen deposition pressure. Fig. 2 shows the in-

600

400

200

I

(“C)

Fig. 1. [Pb]/[Ti] and [Ca]/[Ti] atomic ratios obtained by electron microprobe analysis of the Pb,_,Ca,TiOs films deposited at increasing substrate temperature. A 10% Pb-rich sintered ceramic was used as target. Laser fluence 4 J/cm’. Oxygen deposition pressure, 1 X lo-* mbar.

crease of Pb concentration in the film with increasing oxygen pressure. The Ca concentration remains constant and insensitive to the oxygen pressure. Fig. 3 shows the grazing X-ray diffraction spectra of the films. Below 400°C amorphous films are obtained. Using sintered targets several peaks corresponding to the perosvkite Pb,_.Ca,TiO, phase are observed when the substrate temperature is above 4OO”C, however the preferred crystalline orientation

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I

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Fig. 2. [Pb]/[Ti] and [Ca]/[Ti] atomic ratios obtained by electron microprobe analysis of the Pb, $a,TiO, films deposited at increasing oxygen pressure. A 10% Pb-rich sintered ceramic was used as target, Deposition temperature, 25°C. Laser fluence 4 J/cm’.

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M.J. Martin et al./Applied Surface Science 96-98 11996) 823-826

degree is poor. The orientation degree in the film increases using non sintered target as observed in the comparison shown in Fig. 3. The results displayed in Fig. 3 show little difference in the angular position of (001) and (100) X-ray diffractions of the perovskite phase. According to the results obtained in bulk ceramics this low tetragonality would correspond to a low Pb concentration in the crystalline fraction of the film [l]. We have observed that 200 nm thin fihns deposited at substrate temperatures between 400 and 550°C always show low tetragonality even that according to Fig. I the average Pb concentration in the film is large enough to form films with much larger tetragonality. We believe that the low tetragonality observed is due to the influence of the bottom Pt electrode that strains the crystalline film lattice. In order to investigate this possibility we have prepared films with increasing thickness. Fig. 4 shows the grazing X-ray diffraction of the films

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800

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200

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40 28

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Fig. 4. Grazing angle X-ray diffraction of Pb,_.Ca;TiO, films with increasing thickness. A 10% PbO-rich non sintered ceramic was used as target. Deposition temperature, 550°C. Laser fluence 4 J/cm’. Oxygen deposition pressure, 1 X10-l mbar. The angular position of the X-ray diffraction of the perovskite phase as well as other features related with the Si substrate and Pt electrode are marked for reference.

Pt

L

Pt 800

Si

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28 (deg) Fig. 3. Grazing angle X-ray diffraction of Pb, _,Ca,TiO, films deposited at increasing temperatures using a 10% PbO-rich sintered target (s) and a 10% PbO-rich non sintered target (ns). Laser fluence 4 J/cm?. Oxygen pressure 1 X lo-’ mbar. The angular position of the X-ray diffraction orders of the perovskite phase as well as other features related with the Si substrate and Pt electrode are marked for reference.

prepared. It may be observed that increasing the film thickness up to 600 nm induces a marked shift of (001) X-ray diffraction as corresponding to a larger tetragonality (c/a). For larger film thickness the orientation and crystalline degree of the films decrease. The ferroelectric behavior of the films formed has been investigated by applying a 100 Hz triangular wave of amplitude 2 V. A remanent polarization P, = 1.5 pCcm_* was obtained. Applying consecutive pulses a net polarization of 0.5 p.Ccm-’ was obtained after a relaxation time of 1 s. This value should increase with the applied voltage up to saturation. Retain measurements were carried out using a - 5 V square writing pulse followed by two 3 V reading pulses. After 300 s, the switching polarization of the films is completely relaxed. Reverting the sign of the pulses leads to a larger retain, since 1000 s after the writing pulse the net polarization is 2.6 PC cme2.

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M.J. Martin et al. /Applied Surjke Science 96-98 (1996) 823-826

This asymmetry has been often observed in ferroelectric thin films [5] and is likely due to the presence of space charge inducing an internal electric field [6]. Switching currents were also measured by the pulse sequence method [7]: Several poling pulses are followed by two equal testing pulses of opposite sign, these measurements are also performed with the pulse signs reverted. The results show P, values of 2.8 and 11.8 k,C cm-* respectively and a switching time 2t,,, = 3.4 l.~s, being f,,, the time corresponding to the maximum relaxation current. Again the asymmetry effect is observed although the values of P, are larger than before. This is thought to be due to the smaller time elapsed between the reading pulses. In conclusion crystalline Pb, _ .Ca,TiO, thin films have been prepared on Pt-sputtered Pt/TiO,/SiO,/( 1OO)Si substrates by pulsed laser deposition. The Ca stoichiometry of the film remains equal to that measured in the target, however a Pb loss is observed at substrate temperatures higher than 400°C. 600 nm thick films prepared from non sintered ceramics show an improved fraction of per-

ovskite phase and larger tetragonality. An asymmetric ferroelectric behaviour has been observed.

Acknowledgements This work (MAT93-0095). chamber by the dad Aut6noma edged.

has been supported by CICyT The construction of the deposition SEGAINVES workshop of Universide Madrid is gratefully acknowl-

References [l] J. Mendiola, B. Jimenez, C. Alemany, L. Pardo and L. de1 Olmo, Ferroelectrics 94 (1989) 183. [2] L. de1 Olmo, L. Pardo and J. Mendiola, Ferroelectrics 81 (1988) 289. [3] R.E. Leuchtner and KS. Gmbowski, in: Pulsed Laser Deposition of Thin Films (Wiley, New York, 1994) ch. 20, p. 473. [4] L. de1 Olmo, L. Pardo, B. Jimenez and J. Mendiola, Ferroelectrics 81 (1988) 293. [5] J. Scott, C.A. Araujo, H. Brettmeadows and L.D. McMillan, J. Appl. Phys. 66 (1989) 1444. [6] K. Okazaki, H. Igorashi, K. Nagata and H. Hasegawa, Ferroelectrics 7 (1974) 153. [7] SK. Dey and R. Zuleeg, Ferroelectrics 108 (1990) 37.